Herpes zoster is a viral disease caused by a varicella zoster virus (VZV). Initial infection of VZV is usually in childhood to cause varicella. The infected host may acquire humoral and cellular immunities against VZV to cease proliferation of virus. At the same time, virus may partly be incorporated into the peripheral end of sensory nerve fibers distributed in the skin, ascend the sensory nerve fiber along a retrograde axonal flow, and cause latent infection in ganglion. When the specific cellular immunity against VZV is lowed in the host, virus is reactivated to develop the syndrome of the herpes zoster. The reactivated virus descends the sensory nerve fiber along an antegrade axonal flow to reach the skin and cause zonary exanthema. The exanthema is naturally healed generally within about three weeks.
The zoster-associated pain is generally classified into an acute phase pain and a chronic phase pain. The chronic phase pain is a general problem, which continues over a long term after healing the exanthema. The chronic phase pain is called postherpetic neuralgia, which is considered as neuropathic pain caused by neural degeneration in central and peripheral nervous systems (Non-patent documents 1, 2, and 3). Tricyclic antidepressant, opioid, antiepileptic drugs or the like are used for the present to treat postherpetic neuralgia, and their treatment effects are reported.
On the other hand, the acute phase pain is prodrome or pain in an acute phase, which causes neuralgia-like pain several days before development of exanthema. It includes neuritis pain caused by proliferated virus and inflammatory pain within the area of exanthema. The area of exanthema can be the center of algesthesia, and pain runs along the nerve. The strength of the pain widely ranges from lightly stimulated symptom to heavy pain losing sleep at night. A HSV-1 infected rat has been used as a herpes zoster model. It has been confirmed that the rat can be spontaneously excited without inflammatory syndrome in the spinal posterior root ganglion. It is suggested that acute phase pain may be expressed with spontaneous hypersthenia of the primary neuron cell body (Non-patent document 4).
The acute phase of herpes zoster is treated for the present by using a combination of antiviral agent such as Valaciclovir, Aciclovir, Famciclovir or the like with an analgesic such as NSAIDs. The NSAIDs has an effect on inflammatory pain, but no effect on neuritis-like pain. Further, whole body administration of opioid is sometimes used. It has been known that the above-mentioned pharmacotherapy can have an accelerating effect of healing exanthema, and show a partial effect on acute phase pain, but it cannot suppress the pain completely. Therefore, nerve block treatments, such as epidural block, stellate block, peripheral nerve block has been used in the case that the exanthema and pain are serious (Non-patent document 5). For the reasons mentioned above, it has been desired to develop a method of treating acute phase of herpes zoster pain in view of QOL of the herpes zoster patients.
The present inventors have found that paroxetine, a diazepinedione derivative or the like having a P2X4 receptor antagonism can be used as an agent for preventing or treating neuropathic pain, and filed patent applications (Patent documents 1 and 2).
The patent documents, however, do not clearly describe that the above-mentioned compounds are available as an agent for preventing or treating zoster-associated pain in acute phase.